Senile dementia of the Alzheimer type (SDAT) may be the most common neurologic disorder of late life. As the most frequent basis for nursing home referral and a major, unrecognized cause of death, SDAT represents the central dilemma of aging. Ever later retirement and an aging population further highlight its importance. If published estimates apply, some 1.4 - 4.3 million Americans will be affected by 1990, although prevalence in non-institutionalized persons in the U.S. is presently unknown. Cause is unknown. The clinical disorder affects only man. Early diagnosis is essential in individual care, health planning and studies of pathogenesis. If prophylaxis or therapy becomes available, early identification will be mendatory. Among the practical problems in human study have been the lack of an easily testable definition, simple operational diagnostic techniques, and access to large numbers of potentially affected non-institutionalized persons. Recent collaborative experience has enabled us to evolve a working definition. We have developed and concurrently validated a brief, economic mental function test (MFT) and a serially branching survey diagnostic instrument. We propose to complete their evaluation in a large stable and well-defined retirement community. In the process we will obtain preliminary estimates of the prevalence of SDAT and of depression associated with cognitive impairment among persons aged 65-84. The MFT alone should permit efficient screening in clinical practice and hospital studies. Future studies in this population will include additional descriptive data, evidence for familial or spouse (environmental?) clustering and longitudinal studies to distinguish SDAT from normal aging and to define the natural history of SDAT.